The EM Theremin - Pitch and Volume Antenna: not working.

In ever case you'll have to take off U3. Then you'll have to make sure that all voltages around are correct before putting one back again. I hope that you have been smart enough to use a 16pin socket for it...

If you found 10.95V at pin15, there is for sure something wrong! Either U3 is defective and/or R36 is less than 1Meg and/or R35 is less than 4.7Meg and/or there is a connection problem somewhere. I really wonder if I am the only one to see or sense such things, just by looking at the schematics. It appears to be so basic to me.

The idea when building such complex circuits is to build them block after block and to go to the next block only after you made sure that the preceding block is working correctly. That means that it makes absolutely no sense to connect the VCA preprocessor around U3B before you haven't the volume oscillator and the volume antenna circuit working correctly within their specifications. The latter can be made sure simply by checking the voltage at the anode of D1. As long as you haven't 0V at most frequencies with a clear resonance peak which does not exceed -4V there is even no need to look more far away.

Yes, I did take U3 off. I will check again everything when i have a better testing gear and update if I have anything new. Thanks again for the help, much apreciated!

With U3 taken off, some voltages will (naturally) change. Here the voltages you may expect with U3 in the circuit and in brackets the voltages when you take it off:

pin 1 Iabc A : -10.6V (undefined)
pin 2 Ibias A: 0 to 0.6V (0 to 12V) depends on the brightness pot
pin 3 In+ A : Waveform from mixer from 0 to -0.6V
pin 4 In- A : -0.1V to -0.5V depends on waveform pot
pin 5 Iout A: around 0V
pin 6 V-- : -12V
pin 7 Inbuff A: around 0V
pin 8 Outbuff A: from 0.6V to 1.2V lower than pin 7 (-12V)

pin 9 Outbuff B: undefined not connected
pin 10 Inbuff B: undefined not connected
pin 11 V++ : +12V
pin 12 Iout B: -12V at mute to +12V at full volume (around -10V)
pin 13 In- B: -2V at full volume to +2V at mute
pin 14 In+ B: 0V
pin 15 Ibias B: 0.6V to 2.6V (> 11V)
pin 16 Iabc B: -10.6V (+12V)

All voltages without U3 are correct except pin13 (11,96) [and pin 15 is 10,95 (not >11), but i guess that is not major]

Ok, yes, pin 15 doesn't seem to be the problem but pin 13.

There is a point in the schematic where the Anode of D1 (the side without the ring), C12 and R14 are connected together. Please shorten that point temporarily to ground (that won't damage anything). The voltage at pin13 (without U3) should now go down to ca. 2V. Please report back if it does. 

I grounded that point, but the voltage is still at 11,96V.

With the grounded point, R14 and R35 form a simple voltage divider between +12V and ground:

GND---|R14 1Meg|---Pin13---|R35 4.7Meg|---+12V

It seems that the +12V get via R35 (is it really 4.7Meg, measured?) well to pin13, but that the route between pin13 via R14 (is it really 1Meg, measured?) and ground is interrupted. 

With power off, check both resistors for correct resistance with a multimeter. Then check that you have really 0 Ohms between the temporarily grounded side of R14, 0 Ohms between the other side of R14 and pin13, 0 Ohms between pin13 and the corresponding wire of R35, and 0 Ohms between the other side of R35 and the +12V line.

Then, with power on, check the voltages on both sides of R14 and both sides of R35 and report the 4 values back.

Be prepared to write 100 times :
"I shall not build a theremin if I'm not able to build and fix a simple voltage divider!" ;-)

(sorry for the late reply)

Thierry, I don't think that writing this 100 times will make up for my epic stupidity: R35 was not 4,7M but 4,7K. Feel free to throw stones at me.

I replaced R35 with the correct resistance this time. (All the other values are ok, I assure you)

Now, when I ground R14 (D1 side), the voltage there is 0V as expected. But instead of ~2,1 at the other side of R14 and at the same side of R35, the voltage is ~ -0,4! R35 has 12V at the other side.

Before you bother answering me, give me the weekend to review all the connections again and again, because I don't want to feel like an idiot again.

EDIT: The volume oscillator side still behaves the same (signifficant variations), causing great variations at pin13 (between ~ -1,5 and -10V)

"Thierry, I don't think that writing this 100 times will make up for my epic stupidity: R35 was not 4,7M but 4,7K. Feel free to throw stones at me. I replaced R35 with the correct resistance this time. (All the other values are ok, I assure you)"

"Now, when I ground R14 (D1 side), the voltage there is 0V as expected. But instead of ~2,1 at the other side of R14 and at the same side of R35, the voltage is ~ -0,4! R35 has 12V at the other side."

It may be that the voltage is pulled somewhat down via leakage through R37-D5-R30-R29 towards the -12V line. Temporarily shorten C28 or disconnect one side of D5 to see if the voltage at pin13 (still without the U3 in place) goes up to the correct value. If it does, that might be ok.

"EDIT: The volume oscillator side still behaves the same (signifficant variations), causing great variations at pin13 (between ~ -1,5 and -10V)"

There is definitively still something wrong with the volume oscillator or volume antenna circuit. This has to be fixed before you put U3 back into its place. If not, you will kill it again.

Ok, I shortened C28 and the voltage at pin13 went up to ~1,6V (when R14 is grounded). I checked the connections all around that part, but there doesn't seem to be anything wrong.